This invention relates generally to radio-frequency (RF) filters and, more particularly, to RF filters used at microwave frequencies in various applications, such as communication systems. Filters are necessary components of communication systems and in other RF systems. Typically, a filter includes one or more inductors and one or more capacitors in combination to provide a desired frequency filtering function. Such a filter is often referred to as an LC filter, since the letters L and C are often used in circuits to refer to inductors and capacitors, respectively.
Although the capacitance and inductance parameters of a filter may be uniformly distributed in a physical structure, in practice filters typically employ “lumped” elements connected together to perform the filter function. A capacitor in a lumped LC filter may take the form of a chip capacitor, and a lumped inductor usually takes the form of a coil of conductive material. Coupling these components together is effected by ribbon bonding techniques, which are necessarily very labor intensive and result in a higher cost. An even more serious drawback of lumped element LC filters of the prior art is that the variations in L and C values in typical lumped inductors and capacitors may be greater than 10 percent. The uncertainty of L and C values, and repeatability of extensive ribbon bondings, severely degrade the unit-to-unit tracking performance.
Accordingly, there is a need for an LC filter structure that is not subject to these well known disadvantages of lumped impedance LC filters. The present invention is directed to this end.